The optical design of X-Shooter for the VLT
P. Spanò*a,b, B. Delabrec, A. Norup Sørensend, F. Rigale, A. de Ugarte Postigof, R. Mazzolenic,
G. Saccob, P. Conconia, V. De Caprioa, N. Michaelsend
a
INAF-Oss. Astronomico di Brera, V. E.Bianchi 46, I-23807 Merate (LC), Italy
INAF-Oss. Astronomico di Palermo, P.zza Parlamento 1, I-90134 Palermo,Italy
c
ESO, K.Schwarzschild-str. 2, D-85748 Garching b. Muenchen, Germany
d
CUO-Niels Bohr Inst., J.Maries Vej 30, DK-2100 Copenhagen, Denmark
e
ASTRON, P.O. 2, 7990 AA Dwingeloo, The Netherlands
f
Inst. de Astrofisica de Andalucia, C.Bajo de Huetor 50, E-18008 Granada, Spain
b
ABSTRACT
The overall optical design of X-Shooter, the second generation, wide band, intermediate resolution, high efficiency,
three-arms spectrograph for the VLT, is presented. We focus on the optical design of the three optimized arms, covering
UVB (300-550 nm), VIS (550-1000 nm), and NIR (1000-2300 nm) wavelength ranges, including spectrographs and
pre-slit optics. All spectrographs share the same original “4C” concept (Collimator Correction of Camera Chromatism).
We describe also the auxiliary optics, such as dichroics, acquisition and guiding unit. Performances analysis are
summarized.
Keywords: faint object spectroscopy, wide band coverage, echelle spectrograph, white pupil design, NIR spectroscopy
1. INTRODUCTION
X-shooter is a high-efficiency, wide band, intermediate resolution spectrograph observing simultaneously from 300 to
2300 nm (UBVRIJHK). Light from the telescope Cassegrain focal plane is wavelength-split by two dichroics and sent
to three separate and optimized spectrographs through pre-slit optics.
A detailed optical analysis has been performed to validate the initial concept and verify if instrumental requirements will
be fully matched. Overall description of the project is described elsewhere in these proceedings1,2.
1.1 Selection of the spectrograph design
Different spectrograph designs (ESI-like, UVES-like, 4C-like) have been studied during both feasibility study and
preliminary design review (PDR) phases. After these efforts, a comparative analysis identified the “4C” layout3
(Collimator Compensation of Camera Chromatism, see § 1.2) as the best option for the spectrograph layout.
Main drivers were reduced weight of the optics, reduced weight of mechanics, improvement of the stiffness, reduced
costs, higher efficiency. UVB and VIS spectrographs share a very similar opto-mechanical layout. A 4C-modified
layout has been developed for the NIR spectrograph to match cryogenic requirements.
Pre-slit optics has been designed around a mechanical concept for the backbone structure4 where all three spectrographs
will be mounted. Both UVB and VIS pre-slit arms contains an atmospheric dispersion corrector (ADC).
*
Email: paolo.spano@brera.inaf.it
Ground-based and Airborne Instrumentation for Astronomy, edited by Ian S. McLean, Masanori Iye,
Proc. of SPIE Vol. 6269, 62692X, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.671162
Proc. of SPIE Vol. 6269 62692X-1
1.2 The 4C concept
The idea to compensate camera chromatism through chromatic effect introduced by a Maksutov-type collimator has
been proposed some years ago3. The design of the camera is considerably simplified if it does not need to be
achromatized. In the 4C concept a spherical mirror gives a positive power and a dioptric diverging lens correct
collimator aberrations and also introduces the negative chromatism required to compensate that of the camera.
Moreover, the camera is smaller and less expensive than those required in more classical designs, like in ESI at the
Keck telescope or UVES at VLT.
2. SPECTROGRAPHS
Figure 1 shows the optical layout of the UVB spectrograph. Light from the entrance slit, placed behind the plane of the
figure, feeds an off-axis Maksutov-type collimator through a folding mirror. The collimator consists of a spherical
mirror and a corrector lens (with only spherical surfaces) and is used in double pass. The collimated beam passes
through a prism twice to gain enough cross-dispersion. Main dispersion is achieved through an echelle grating. After
dispersion, the collimator creates an intermediate spectrum near the entrance slit, where a second folding mirror has
been placed. This folding mirror acts also as field relay mirror. Then a dioptric camera (1 aspherical surface) reimages
the cross-dispersed spectrum onto a tilted detector. Collimated beam size is 10cm, while camera optics are smaller than
Ø80 mm.
NIR spectrograph is shown in Figure 2. The 4C concept has been modified to decrease overall size through a twomirrors collimator. A three elements camera with mild aspherical surfaces will enhance overall efficiency, minimizing
the possibility of ghosts. Cross dispersion is realized through three prisms in series, one 35° Silica prism and two 22°
ZnSe prisms. Such a combination gives a dispersion more uniform than that one of a single prism, maximizing the
spectral coverage for a fixed detector area.
Spectrograph main parameters are summarized in table 1.
Folding mirror I
Folding mirror 2
Prism
/_
Entrance slit
(out of the plane:
Main minor
---
-Grating
Cam era
Corrector lens
Detector CCD 2Kx4K
(tilted surface)
Fig. 1. The UVB spectrograph optical design. VIS arm has a very similar design. Collimated beam diameter is 10cm.
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Table 1. Spectrograph main parameters.
UVB arm
Collimator Focal Ratio
Slit height
Collimated beam diameter
Spectral range (λλ)
Resolution-slit prod. (Rϕ)
Camera free diam. (max)
Camera magnification
Camera output Focal Ratio
Detector
Detector scale
Grating
Prism
F/6.5
12 arcsec = 3.06 mm
100 mm
300-550 nm
4500 (arcsec)
80 mm
0.41
F/2.64
4Kx2K CCD, 15 µm pixel
105 µm/arcsec (main
dispersion) = 7 pixel/arcsec
180 gr/mm, 41.8° blaze
angle, 2.2° off-plane angle,
orders 14-24
60° Silica
VIS arm
NIR arm
F/6.5
12 arcsec = 3.06 mm
100 mm
550-1000 nm
7000 (arcsec)
80 mm
0.42
F/2.7
4Kx2K CCD, 15 µm pixel
111 µm/arcsec (main
dispersion) = 7 pixel/arcsec
99.4 gr/mm, 54.0° blaze
angle, 2.0° off-plane angle,
orders 17-30
49° Schott SF6
F/13.4
12 arcsec = 6.36 mm
85 mm
1000-2300 nm
4500 (arcsec)
70 mm
0.2
F/2.1
2Kx1K IR array, 18µm pixel
90 µm/arcsec (main
dispersion) = 5 pixel/arcsec
55.0 gr/mm, 46.1° blaze
angle, 1.8° off-plane angle,
orders 11-26
35° Infrasil + 2x22° ZnSe
echo lie
sift
prisms
MG
corrector
lens
MT
Camera
lenses
Fig. 2. NIR spectrograph layout.
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detector
2.1
Spectral formats
Spectral formats have been computed via ray-tracing of the optical model of each spectrograph. Spectral orders are
curved. Line tilt and inter-order separation have been optimized to have enough free pixels between adjacent orders, and
to ensure well calibrated reduced spectra.
Figure 3 shows a simulated ThAr spectrum.
uves_rebin.c
Fig. 3. Simulated ThAr spectrum in the VIS. The box shows a 500 x 500 window centered on order 24 near 680 nm.
2.2
Performances analysis
Extended analysis have been made to optimize overall performances, including image quality, energy concentration,
sensitivity and tolerances of each optical element, thermal analysis, ghosts and stray-light analysis. Some of these
analysis have driven further opto-mechanical analysis such as flexures and environmental analysis to check instrument
stability.
Coatings will maximize overall efficiency. Figure 4 shows the simulated efficiency from Cassegrain focal plane to the
detector (included). Table 2 summarizes average efficiency for each arm. X-Shooter will be one of the most efficient
spectrographs on 8m-class telescopes, especially below 370 nm.
Table 2. X-Shooter efficiency
Spectrograph
UVB
VIS
NIR
Average blaze efficiency
42%
36%
28%
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Average min. efficiency
22%
22%
16%
DA
OIYV
Or
300
\f\rv
350
400
450
500
550
0.5
0-4
U
0-3
01
w
C
550
01
-02
650
850
750
AR/
V
V
'I
950
'1
\/
0_
1000
1500
2000
2500
WaeIength
Fig. 4. Expected efficiency of UVB (top), VIS (middle), NIR (bottom) spectrograph arms from telescope focal plane to
detector. Ripples are due to the grating efficiency.
3. RELAY OPTICS
Light from the Cassegrain focal plane feeds three different spectrographs, located onto a common backbone. Different
wavelengths are split via two dichroics, placed after the Cassegrain focal plane in the divergent beam, and then different
relay optics reimages the telescope focal plane onto the spectrograph entrance slits. UVB and VIS relay optics are based
onto lenses, while the IR relay optics consists of only mirrors.
NIR arm will consist of four mirrors, two (one spherical, one cylindrical) will be outside the cryostat (warm optics) and
will reimage telescope focal plane onto the NIR spectrograph slit, and other two flat mirrors will be inside of the
cryostat (cold optics).
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Cassegrain focal plane
UVB arm
ADCs
VIS arm
dichroics
IR warm optics
IR arm
cryostat window
IR cold optics
Fig. 5. The pre-slit optics.
2.1
Dichroics
Dichroics have been optimized to maximize efficiency in each arm, minimizing the width of the transition region. The
first dichroic reflects light towards UVB spectrograph and transmits light to the VIS and NIR ones. The second dichroic
reflects light towards the VIS spectrograph and transmits light to the NIR spectrograph.
2.2
Atmospheric Dispersion Correctors
X-Shooter suffers a very strong atmospheric dispersion due to its very wide wavelength range. Efficient correction
cannot be obtained through one device only. NIR atmospheric dispersion is about 0.4 arcsec at 60° zenith angle, then no
correction will be foreseen. Two different ADCs have been optimized for the other two arms, minimizing correction
residuals having un-deviated wavelengths at 405 (UVB) and 633 (VIS) nm.
ADCs consist of two counter-rotating double prisms cemented onto two doublets, reducing air-glass interfaces. These
integrated optics will act as focal reducers to match telescope and spectrograph focal ratios. Small field lenses, located
near the spectrograph slits, will match pupils.
2.3
Slit losses
Light losses onto the entrance slit due to thermal effects (both from backbone and pre-slit optics) have been modelled.
The worst case corresponds to the minimum entrance slit, that was setup at 0.6”. Two cases have been considered: (a)
seeing 0.6” (FWHM), slit width 0.6”, (b) seeing 1”, slit width 0.6”. A simple analytical approach gives us an estimate
for the minimum slit loss for a perfect optical system, with the slit perfectly centred upon the PSF and extended
infinitely in one direction. Slit loss is 24% for the first case, and 48% in the second one. Ray-tracing computations have
Proc. of SPIE Vol. 6269 62692X-6
been carried out. Figure 6 shows results from this analysis. The effect of defocus is not significant at 5% level. No
refocus is foreseen during observations.
SLIT IMAGE (seeing FWHM = 0.6 arcsec)
SLIT IMAGE (seeing FWHM = 1.0 arcsec)
Fig.6. Illumination maps at the UVB entrance slit for different temperatures and seeing conditions.
4. AUXILIARY OPTICS
3.1
Acquisition and Guiding Unit
Acquisition and guiding system will implement several functions. It will allow to detect and centroid objects onto
entrance slits. An artificial star (pinhole), materializing the entrance focal plane, can be placed onto the entrance focal
plane for maintenance purposes. A pellicle beam splitter will allow to use the A&G camera as a slit viewing camera.
Different filters will select different entrance slits. Finally, a three slices 1.8”x4” integral field unit (IFU) will be
available5.
Cassegrain focal plane
Fig. 7. Acquisition and Guiding camera optical layout.
3.2
Pupil viewer
A pupil imager would be convenient, not only for aligning the A&G camera, but also for aligning the arms of the
spectrograph by using the slit viewer pellicle. Two lenses are required: one placed on the A&G side of the pellicle, and
one on the A&G filter wheel. To view each of the three spectrograph pupils, a lens and filter sandwich is selected in the
A&G filter wheel. The same pellicle lens can be used for all three arms.
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Fig. 8. Pupil viewing optics.
3.3
Dichroic viewer
An increase in infrared background can be expected due to dust settling on the first dichroic (DC1). The amount of dust
on DC1 can be monitored by using the slit viewing pellicle and focusing the A&G camera on DC1 by inserting optics
(lens/filter) in the filter wheel.
Fig. 9. The A&G camera acting as dichroic viewer.
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S. D’Odorico, et al.:2004, SPIE 5492, 220
S. D’Odorico, et al.: 2006, these proceedings
B. Delabre, H. Dekker, S. D’Odorico, F. Merkle: 1989, SPIE 1055
N. Michaelsen, et al.: 2006, these proceedings
I. Guinouard, et al.: 2006, these proceedings
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